Furthermore, our body temperature is not a constant. Rather, it fluctuates predictably over the course of the day. Body temperature peaks in the late afternoon and hits its nadir in the early morning hours. This circadian rhythm can lead to variations of as much as 1 degree higher or lower. Thus, an elevation of 2 degrees over one’s normal early-morning nadir, which meets a definition of fever, can easily fall in the 99 point something range.

Choice of thermometers can also add to the confusion. A rectal thermometer is the gold standard, as it comes closest to approximating the body’s core temperature. Other types of thermometers, such as oral thermometers, tympanic or ear thermometers, or forehead thermometers, are more convenient but may yield lower readings.

In addition, normal temperatures tend to vary among certain groups. Women tend to have a slightly higher basal temperature than men. The elderly tend to have lower temperatures than younger people. And other people may just be outliers from the norm.

Your best bet is to determine your baseline temperature. To do this, measure your temperature when you are feeling well. Do this using the same thermometer at the same time of day for several days, and record the results to get an average temperature. Repeating your measurements helps maximize accuracy.

Remind your doctor of your baseline reading. And if your doctor says you don’t have a fever, remind him or her of the C.D.C.’s definition that fever is present when the patient “feels warm to the touch or gives a history of feeling feverish.”